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Abstract

The Ni2+ complexes with hexadentate ligands containing two 6 methylpicolinamide groups linked by ethane-1,2 diamine (H2dedpam) or cyclohexane-1,2 diamine (H2chxdedpam) spacers were investigated as potential contrast agents in magnetic resonance imaging (MRI). The properties of the complexes were compared to those of the analogues containing 6 methylpicolinate units (dedpa2 and chxdedpa2 ). The X ray structure of the [Ni(H2dedpam)]2+ complex reveals a six-coordinated metal ion with a distorted octahedral environment. The protonation constants of the dedpa2 and H2dedpam ligands and the stability constants of their Ni2+ complexes were determined using pH-potentiometry and spectrophotometric titrations (25 ºC, 0.15 M NaCl). The [Ni(dedpa)] complex (logKNiL = 20.88(1)) was found to be considerably more stable than the corresponding amide derivative [Ni(H2dedpam)]2+ (logKNiL = 14.29(2)). However, the amide derivative [Ni(H2chxdedpam)]2+ was found to be considerably more inert with respect to proton-assisted dissociation than the carboxylate derivative [Ni(chxdedpa)]. A detailed 1H NMR and DFT study was conducted to assign the 1H NMR spectra of the [Ni(chxdedpa)] and [Ni(H2chxdedpam)]2+ complexes. The observed 1H NMR paramagnetic shifts were found to be dominated by the Fermi contact contribution. The amide resonances of [Ni(H2chxdedpam)]2+ at 91.5 and 22.2 ppm were found to provide sizeable chemical exchange staturation transfer effect, paving the way for the development of NiCEST agents based on these rigid non-macrocyclic platforms.